A histogram is very similar to a bar graph in which each bar represents some class or element (for example, a score on an IQ test). The primary difference between a bar graph and a histogram is that the bars in the histogram actually touch each other to show that there are no gaps in between the classes. The bars in a bar graph have space in between them.
Let's take an IQ test as an example of a histogram. The scores would appear along the X axis (the bottom/horizontal axis) and the frequency would appear along the Y axis. We might have a range of IQ scores on the X axis from 50 to 200 and a bar at each class (each score) representing how many people received that score on the IQ test. Each bar would touch the other bars next to it to show that there is no gap between the scores.